Polyester resins and, in particular, polyethylene terephthalate, polyethylene naphthalate, polytrimethylene terephthalate and polytetramethylene terephthalate are excellent in mechanical, physical and chemical properties, and thus the polyesters are widely utilized for fibers, films and other formed materials. For use in nonwoven fabrics in particular, polyesters are known to have excellent mechanical strength, dimensional stability, heat resistance and light resistance.
A polymer for fibers, as mentioned above and, for example, a polyethylene terephthalate, is usually produced by preparing ethylene glycol ester of terephthalic acid, and/or a polymer with a low polymerization degree of the ester, and reacting the products in the presence of a polycondensation catalyst at reduced pressure while the products are being heated, until the polymer has a given polymerization degree. Moreover, other polyesters such as a polyethylene naphthalate, a polytrimethylene naphthalate and a polytetramethylene terephthalate are produced by methods similar to that explained above.
For some types of polycondensation catalysts, it is well known that the quality of the polyesters thus obtained greatly depends on the catalysts. Antimony compounds have been most widely used as polycondensation catalysts for polyethylene terephthalate.
However, when an antimony compound is used, continuous melt spinning of polyester over a long period of time causes sticking and deposition of foreign matter around the periphery of the spinneret (hereinafter merely referred to as spinneret foreign matter). As a result, bending of molten polymer streams takes place to cause the problems that fluff formation, yarn breakage or uneven fiber physical properties appear. For a filament yarn of which the fiber physical properties must be utilized as much as possible, a solution to the above problems has been particularly desired.
Use of a titanium compound such as titanium tetrabutoxide for the purpose of avoiding the problem has been known. However, when such a compound is used, the polymer thus obtained shows poor thermal stability and drastically deteriorates during melting. Accordingly, fibers having a high mechanical strength are hard to obtain. Moreover, the thus obtained polyester is itself yellowed to cause the problem that the finally obtained fibers have an unsatisfactory color tone.
The following procedures have been disclosed as means for solving such problems: reaction products obtained by reacting a titanium compound with trimellitic acid are used as a catalyst for producing the polyester (see, e.g., Reference Patent 1); products obtained by reacting a titanium compound with a phosphorus acid ester are used as a catalyst for producing the polyester (see, e.g., Reference Patent 2). Although these methods surely improve the melt heat stability of the polyester to a certain degree, the effects of improvement are inadequate, and the color tone of the polyester resin thus obtained must be improved. Moreover, use of a complex of a titanium compound and a phosphorus compound as a catalyst for the production of a polyester has been proposed (see, e.g., Reference Patent 3). Although the melt heat stability is improved to a certain degree when the method is employed, the effect is not sufficient, and the color tone of the polyester thus obtained must be improved.
The cited references mentioned above are described below.
[Reference Patent 1]
Japanese Examined Patent Publication (Kokoku) No. 59-462658
[Reference Patent 2]
Japanese Unexamined Patent Publication (Kokai) No. 58-38722
[Reference Patent 3]
Japanese Unexamined Patent Publication (Kokai) No. 7-138354